1. Field of the Invention
This invention relates to a photometric amplifier for the photometric circuit of cameras in which a photodiode is employed as a light receiving element.
2. Background of the Invention
A camera has a photometric circuit for converting the luminance of an object into electrical data. In general, the photometric circuit is a so-called "photometric amplifier" which is made up of a light receiving unit such as a photoelectric coversion element or photovoltaic element and an amplifier for photoelectric conversion data.
FIG. 1 is a circuit diagram showing one example of a conventional photometric amplifier. The photometric amplifier includes an integrating circuit formed by connecting a capacitor 2 between the inverting input terminal and the output terminal of an operational amplifier 1. A photovoltaic element, namely, a photodiode 3 is connected between the inverting input terminal and the non-inverting input terminal of the operational amplifier 1.
In the photometric amplifier, the photodiode 3 receives light from an object and subjects it to photoelectric conversion to output a photocurrent i and the photocurrent i is charged in the capacitor 2. As a result, an input voltage v.sub.i at the inverting input terminal increases in the positive direction with respect to a reference voltage V.sub.ref at the non-inverting input terminal. As a result, an inverted output voltage V.sub.o, according to the differential input voltage, is output as photometric data.
The photometric data is operated upon with exposure factors such as an aperture value and a film sensitivity to obtain shutter time data. However, in general, the photometric data is processed as logarithmically converted compressed data in order to cover a wide range of object luminacents.
A camera having a first photometric amplifier for measuring the brightness of an object under natural light and a second photometric amplifier for measuring the brightness of an object illuminated by an artifical light source from an illuminating unit such as a flash light has been proposed in the art. In this camera, it is unnecessary to subject the output photometric data of the second photometric amplifier to logarithmic compression. Therefore, the photometric data is operated upon with an exposure factor or a film sensitivity ISO.
What is shown in FIG. 1 is one example of the second photometric amplifier in which the photometric data and film sensitivity data V.sub.S are subjected to comparision in a comparator 4, which provides an output V.sub.x such as shutter time data.
An electronic shutter camera using a microcomputer has been developed. In the camera, a film sensitivity ISO is stored as digital data in memory. The digital data corresponding to the film sensitivity thus set is applied to a D/A (digital-to-analog) converter, so that it is output as the film reference voltage V.sub.S of the above-described comparator 4.
In the above-described photometric amplifier, the output voltage V.sub.o is compared, as it is, to a film reference voltage V.sub.S corresponding to the ISO film sensitivity. Therefore, the film resolution of the film reference voltage V.sub.S introduces difficulties.
The following Table 1 shows resolution examples of the reference voltage V.sub.S mentioned above:
TABLE 1 ______________________________________ (V.sub.s : millivolts (mV)) V.sub.s V.sub.s Resolution Example 1 Resolution Example 2 ISO ______________________________________ 40 320 50 20 160 100 10 80 200 5 40 400 2.5 20 800 1.25 10 1600 ______________________________________
In resolution example 1, the maximum voltage 40 mV is a reference value. As the ISO value increases, the voltage is decreased, for instance, to 1.25 mV. It is rather difficult to form memory means for digital data providing such a small voltage value.
In resolution example 2, the minimum voltage 10 mV is a reference value. As the ISO value decreases, a high voltage such as for instance 320 mV is required. Such a high voltage is not suitable for the operation of digital memory means because the voltage value which can be handled by the digital memory means is limited.